Chlorofluorocarbons and ozone

Chlorofluorocarbons and ozone First plenary lecture at Jekyll Island meeting

By Mack McFarland

Chlorofluorocarbons (CFCs) were developed in the 1930s as safe alternatives to ammonia and sulfur dioxide refrigerants (ammonia is &"able and both are toxic). Since then the uses of CFCs have teen expanded to include air conditioning, cleaning of critical electronic and mechanical components, and expansion of plastics for energy- efficient foams. Because of the many essential needs they serve, CFC use should be restricted only if there is sound scientific justification for doing

CFCs have proved to be one of the most useful class of compounds ever developed because of several des ib le properties mostly related to their chemical stability. CFCs promote worker and consumer safety because they are nontbmmable, noncorrosive, nonex- plosive, and very low in toxicity. They can be used in a wide variety of applications because of physical properties including convenient vapor pressure and temperahue characteristics, low vapor phase thermal conductivity, desirable solubility characteristics, com- patibility with many construction mate- rials, and high stability. These properties have led to the widespread use of CFCs in consumer products.

One of the more critical uses of CFCs is in refrigeration of perishable foods and medical supplies. In the

so.

United States, about 75% of the food supply depends on CFC refrigerants for processing, storage, or distribution. Blood, medicines, and donated organs are stored and transported under refrigeration.

Other uses are also valuable to society. Air conditioning of our offices and houses leads to comfort and increased productivity. The reliability of electronic and mechanical components for communications equipment, com- puters, and navigation and conml in- struments for aircraft depends on CFC cleaning agents. Because of their low vapor phase thermal conductivity, CFCs contribute to the efficiency of plastic insulating foams for refrigera- tors, freezers, buildings, and refriger- ated railway cars and trucks; even if the foams could be expanded with air or carbon dioxide, the thermal efficiency would be reduced by about a factor of two.

CFCs provide significant advantages in uses that many consider nonessen- tial. Aerosol propellants are one exam- ple. In spite of the fact that they are more expensive than other alternatives, CFCs were the propellant of choice for many applications because they are nodamnable, very low in toxicity, and provide efficient dispersion. In the United States, their use as a propellant was banned in 1978 except for a few applications. CFCs are used to expand plastic foams for packaging and cush- ioning for safety and health reasons. The low toxicity and nonflammability of CFCs make work places safer, and their high stability eliminates possible contributions to the formation of photo- chemical oxidants in urban areas.

Estimated global production and consumption by application are shown in Figure 1. Total consumption increased until 1974, when the owne depletion theory was 6rst proposed; it decreased

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and then remained approximately constant until the mid-1980s when it again began to increase. The use pattern changed significantly between 1974 and 1988. In 1974 CFCs were used primarily as aerosol propellants, but with an almost total elimination of that application in the United States and decreases elsewhere, they are now used mainly as refrigerants and blowing agents.

Although developed countries currently account for 85-90% of global use, consumption rates are increasing in developing countries as they strive to improve their standard of living. There is some concern that developing countries might choose to continue to use CFCs outside the Montreal protocol agreement and become the major consumer as developed countries reduce their consumption.

History of ozone h e The chemical stability of CFCs that

leads to the desirable safety characteristics also contributes to environmental concerns. There are no known destruc- tion mechanisms for the CFCs in the lower region of the atmosphere, the tro- posphere. Thus, once released into the troposhere, they will remain there until transported to the stratosphere and de- composed by solar ultraviolet radiation.

The first evidence of possible concern about these compounds was Love- lock‘s measurement of the atmospheric concentration of CFC-11 (Lovelock,

1971). A c g q ” s o n of estimated releases of the compound with its concen- hation indicated that very little, if any, had been destroyed. This led to studies of the atmospheric fate of the CFCs.

In 1972, Du Pont invited CFC prc- ducers to a “Seminar on the Ecology of Fluorocarbons.” Quoting from the invi- tation written by Ray McCarthy: “Fluorocarbons are intentionally or ac- cidentally vented to the atmosphere worldwide at a rate approaching one billion pounds per year. These compounds may he either accumulating in the atmosphere or returning to the sur- face, land or sea, in the pure form or as decomposition products. Under any of these alternatives it is prudent that we investigate any effwts which the compounds may produce on plants or ani-

involved ih &e catalytic ozone destruc- tion cvcle had never been observed in ’

FIGURE 1 Estimated world pmductii and msumptlon 0fchbmRuomcarbOns (a) Production m

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- low

500

I 1980 Year

(b) Consumption (by application) 1974 (2025 million Ibs) 1988 (2510 million Ik)

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mals now or in the future.” As a result of that industry sympo-

sium, a research program was estab lisbed to investigate the fate and impact of CFCs in the atmosphere. Nineteen companies formed the Chemical Manu- facturers Association’s Fluorocarbon Program Panel. The panel has funded well over $20 million in research at ac- ademic and government laboratories worldwide, including partial support of the recent Arctic and Antarctic expeditions.

Lovelock’s measurements also initi- ated the researcb of S h e r w d Rowland and Mario Molina into the atmospheric fate of CFCs. Their study resulted in the ozone depletion theory published in Nafure in 1974 (Molina and Rowland, 1974). Stated briefly, they postulated that CFCs would remain in the atmosphere until transported to the shato- sphere where they would be photo- lyzed, releasing chlorine atoms. Then, through a series of catalytic reactions, the chlorine atoms might cause a reduction in the total amount of ozone. Be- cause ozone absorbs most of the solar ultraviolet radiation in the 280-31Onm region (UVb), a net ozone decrease could lead to increases in UVb. Al- though many uncertainties remain r e gardiig potential effects, increases in UVb could lead to adverse effects on plants and animals.

There were insufficient data to test the theory at the time it was proposed. For examvle. the chlorine comwunds

erowls Refrigerants

the s&tosphere. Led by government agencies but with

significant input from industry, scientists from government, academia, and industry undertook the enormous task of developing the science with the goal of predicting future ozone amounts. Developing a predictive capabdity is important because, once in the atmosphere, the CFCs remain there for about 100 years. If policies controlling the use of CFCs were not implemented until there was evidence of harm to plants or animals, the effect would persist for decades. However, the available scientific information indicated that any significant ozone depletion would probably not occur for decades, if at all, and time was available to develop the science.

Then as now, the industry position was that any CFC policies should be based on sound scientific information. Du Pont’s position on the issue was based on its environmental policy adopted in the late 1930s. That policy commits Du Pont to “determine that each product can be made and disposed of safely and consistent with appropri- ate safety, health and environmental

1204 Environ. Scl.Technol., Vol. 23, NO. 10,1989

quality criteria.” For CFCs, this meant that Du Pont’s position was based on the best available science.

From 1975 through the early 1980s, global production of CFCs was almost constant, and the available information indicated that there had been no persistent trend in ozone and none was ex- pected. However, because there was still significant scientific uncertainty, both research and assessments of the need for controls was continued. Avail- able information indicated that significant ozone changes were unlikely un- less there were a significant increase in consumption of CFCs. This information is summarized in the following sentences taken from a 1985 international assessment report (World Mete- orological Organization, 1986): “Time dependent scenarios were performed using one-dimensional models assum- ing C02, CH4, and N20 annual growth rates of 0.5 %, 1 % , and 0.25 %, respec- tively, in conjunction with CFC growth rates of 0%, 1.5% and 3% per year. The ozone column effects are relatively small ( < 3 % over the next 70 years) for CFC increases of I 1.5 % per year, but with CFC growth rate of 3% per year the predicted ozone depletion is 10% after 70 years and still rapidly increasing.”

During the mid-l980s, global consumption of CFCs began to increase. Furthermore, forecasts indicated that demands for CFCs would continue to increase, primarily because developing countries would require CFCs to provide essential services such as refrigeration and insulation. These growth forecasts coupled with available scientific information indicated that global limits to CFC consumption were war- ranted. We at Du Pont came to this conclusion after one of our periodic reviews of the science during the summer of 1986. Led in September 1986 by Du Pont and the Alliance for Responsible CFC Policy, the worldwide CFC industry advocated international efforts to limit growth of CFC emissions.

The Montreal Protocol Because CFCs are used worldwide

and because they have equal potential to affect ozone regardless of where they are emitted, any effective regulations to control emissions must be global. Therefore, the CFC issue requires international solutions,

In 1977 the United Nations Environ- ment Programme (UNEP) formed the Coordinating Committee on the Ozone Layer, which prepared periodic scientific assessments. In 1981 UNEP formed an ad hoc group to consider development of a global convention. A result of these international efforts was the Vienna Convention for the Protec-

tion of the Ozone Layer adopted in March 1985. The convention was de- signed so that protocols could be added requiring specific control measures, and plans were outlined for a series of workshops to further evaluate the need for such controls.

During December 1986 international negotiations on the development of regulations began. With the forecasts of growth in the use of CFCs and the science indicating that significant growth could lead to appreciable ozone depletion, there was a growing consensus to limit global consumption of CFCs. The basis for consensus was a shared goal of protecting the environment, commitment to advancing scientific understanding, and agreement that regulations should be based on sound scientific information. The result of the negotiations was the signing of the Montreal Protocol on Substances That Deplete the Ozone Layer in September 1987. The protocol was ratified by the required number of countries and took effect on January 1, 1989.

The protocol requires developed countries to reduce their consumption of CFCs to 50% of 1986 rates over a 10-year period. Developing countries are allowed to increase consumption for 10 years (their consumption cannot exceed 0.3 kg/person). Given the scientific information incorporated in the atmospheric models used for forecast- ing ozone concentrations, it could be argued that these reductions provided a margin of safety for ozone protection. Model forecasts based on these consumption reductions indicated that ozone might actually increase over the next century. However, there had been two developments in the science that provided the basis for an argument that the reductions might be justified.

During 1985 an article was published reporting significant and unexpected ozone decreases over the Antarctic con- tinent each spring since the mid to late 1970s (Farman et al., 1985). There were also preliminary reports based on measurements by a satellite instrument that global ozone amounts were de- creasing at the rate of about l % per year. Although ground-based measurements indicating no persistent change in ozone conflicted with the satellite results, and the cause of neither the Antarctic ozone decreases nor the possible global ozone decreases were un- derstood, both were clearly cause for concern and further investigation. The Antarctic ozone decreases led to a series of measurement campaigns (two ground-based campaigns, the National Ozone Expeditions; and an aircraft campaign, the Airborne Arctic Ozone Experiment) to establish the cause. The reports based on the satellite ozone

measurements led NASA to organize an international assessment of ozone data, the International Ozone Trends Panel.

Basis for a phase-out The results of the Antarctic experi-

ments and the findings of the Ozone Trends Panel significantly changed our understanding of the processes that control ozone and the involvement of CFCs in those processes. This new understanding provides the scientific basis for a phase-out of the production and consumption of CFCs.

The Executive Summary of the Ozone Trends Panel Report (NASA, 1988) was released on March 15, 1988. Three major conclusions of the report are:

There have been small but measur- able decreases in the amounts of ozone at high northern latitudes during winter over the last 17 years. The decreases appear not to be due to known natural effects, and although the cause or causes have not been established, the pattern of decreases fits the calculated pattern of decreases that CFCs might cause. Also, if the chemistry contributing to the Antarc- tic decreases is having an effect in the Northern Hemisphere, the effect would be predicted to occur at high latitudes during winter and early spring. “The weight of evidence strongly in; dicates that man-made chlorine spe- cies [primarily CFCs] are primarily responsible for the observed decreases in ozone within the polar vor- tex [the region of the Antarctic ozone decreases] .” “While the column ozone depletion is largest in the Antarctic springtime, normalized TOMS [Total Ozone Mapping Spectrometer] data indicate that total column ozone has decreased since 1979 by more than 5 % at all latitudes south of 60 degrees throughout the year. At this time it is premature to judge if this is caused by dilution of the air from the region of very low ozone, a changed meteo- rology, or some other unidentified phenomenon. However, at least some of the decrease is likely due to dilution [of air from the region of the Antarctic ozone decreases] .” Of all the findings, the most signifi-

cant is the discovery of a mechanism capable of converting inactive forms of chlorine into active forms. Once CFCs reach the stratosphere, photolysis by solar UV releases their chlorine atoms. Through a series of reactions, these chlorine atoms are converted back and forth between active forms that can participate in the catalytic reactions de- stroying ozone and inactive forms.

Environ. Sci. Technol., Vol. 23, No. 10, 1989 1205

Throughout the region of maximum owne concentrations the ratio of inactive to active forms is about 100: 1.

The results of the measurement campaigns and laboratory studies have shown that reactions on particles of water ice and on nitric acid and water within the cold region of the Antarctic stratosphere during winter and spring can convert the inactive forms of chlorine into the active forms. Measure- ments of the primary active form of chlorine, chlorine monoxide, over Ant- arctica during the time of observed ozone decreases show that its concentrations are elevated by about a factor of 100 above what would be predicted based on previously accepted chemical mechanisms (Anderson, 1989). These concentrations are large enough to account for at least most of the Observed ozone losses during the Antarctic spring.

The combination of these new results and projections of atmosphere concentrations of chlorine from CFCs that could occur with the current control provisions of the Montreal Protocol leads to the conclusion that it would be prudent to take linther steps. Calcu- lated atmospheric chlorine from CFCs based on six assumptions of future consumption is shown in Figure 2. In all cases, it is assumed that worldwide compliance to control measures is achieved. Curve A shows chlorine under the current provisions of the Proto- col and curve B shows the effect of moving the required reductions for- ward in time by one control period-the curves are essentially the same. Curve C shows the effect of an immediate reduction of CFC consumption by 85 %- the reduction required to stabilize chlorine from CFCs. Curves D, E, and F show the effect of the timing of an

eventual 95% reduction of emissions. This analysis shows that if the goal is a reduction in the contribution of chlorine from CFCs, the degree of reduction is much more important than the timing of that reduction.

Based on the new information, on March 24, 1988, Du Pont announced a commitment to an orderly transition to the total phase-out of CFC praiuction. After further evaluation of the time required for a transition, we have stated that the phase-out would occur as m n as possible but not later than the turn of the century. An orderly transition is required to allow the time necessary for the development of alternative compounds and technologies so that the essential needs currently ~ ~ e d by CFCs continue to be met.

An important provision of the protocol calls for periodic reviews of the ad- equacy of the control provisions every four years beginning in 1990. Based on those reviews, the parties to the protocol have the option of modifying the controls. International reviews of the scientific, environmental, technical, and economic aspects of the issue are under way and should be complete by September 1989. The parties to the protocol will meet in April 1990 to de- cide on any changes in the control provisions. Even though many of the details will need to be worked out, there appears to be an international consensus that the use of CFCs should be eliminated as substitutes become available.

Alternatives to CFCs A variety of options will probably be

implemented to achieve a phase-out of CFCs. Dn Pont’s analysis of methods that might be employed in the year Zoo0 to replace CFCs is based on feed

back from customers in over 100 countries and on Du Pont’s experience as the world’s largest CFC producer (about one-fourth share of global production, with manufacturing sites in seven countries). The options include conservation, not-in-kind substitutes (i.e., alternatives outside the fluorocarbon family), hydrofluorocarbons (HFCs), and hydrochlorofluorocarbons (HCFCs). ’ ConsAation includes better mainte- nance practices to prevent leaks from refrigeration and air conditioning equipment, recovery during servicing of that equipment, and recovery and re- cycling of material used for cleaning. It appears likely that conservation programs currently being implemented for CFCs will be continued for the alternatives. These improved conservation practices could decrease demand for CFC alternatives by about 30%.

Most of the aerosol propellant uses will probably be supplied by other products as happened in the United States during the 1970s. Nonfluomcar- bon alternatives will also be used as cleaning and blowing agents. Another 30% of CFC demand will probably be met by not-in-kind alternatives by 2ooO.

At this time it appears that the best options for meeting remaining demands are HFCs and HCFCs. These compounds appear to retain many of the desirable safety and performance characteristics of the CFCs, but, because they contain hydrogen, they are decom- posed in the troposhere through reac- tion with hydroxyl. This decomposition shortens the atmospheric lifetimes of HFCs and HCFCs to 2-20 years, de- pending on the compound, compared to about 100 years or longer for the CFCs. Because the HFCs contain no

1206 Environ. Scl. Technol., Vol. 23, No. 10, 1989

chlorine, they have no potential to deplete ozone. The tropospheric decomposition of the HCFCs limits the amount of chlorine that can reach the stratosphere and reduces their ozone depletion potentials to only 2-10% of those of CFCs.

Another advantage of the shorter lifetimes of these compounds is a reduction in global warming potential compared to CFCs. Concern has been expressed over the 20% contribution of the CFCs to calculated global warming during the 1980s. With their reduced lifetimes, HFCs and HCFCs would contribute about 90% less on a pound- for-pound basis. Considering that only about 40% of the CFC demand will be satisfied with these compounds, over the next century they would contribute only about 4% as much toward global warming as CFCs would if their use were to continue.

We project that about 10% of the demand for CFCs could be met with HFCs in 2000, primarily in the refrigeration applications. Elimination of the chlorine from these compounds increases their vapor pressures and decreases their solubility properties compared to CFCs. These two property changes limit their use for many applications.

The HCFCs appear to be the best option for meeting the remaining 30% of CFC demand. The primary uses of these compounds will probably be in expanding plastic foams for insulation, cleaning of critical electronic and metal components, refrigeration, and air conditioning.

Industry is aggressively developing and testing all of these options to achieve a rapid but orderly transition to the phase-out of CFCs. A critical part of these programs is safety and environmental testing. Three industry- sponsored programs are under way to ensure that the options employed do not pose other problems. Two programs (Program for Alternative Fluorocarbon Toxicity Testing I and II) are doing the studies required to ensure worker and consumer safety. Another program (Al- ternative Fluorocarbon Environmental Acceptability Study) is being sponsored to ensure that the products are environmentally acceptable. Although sponsored by 15 potential producers of alternatives, leading scientists from around the world are conducting the study under the guidance of Robert Watson of NASA.

Conclusions Many questions remain regarding the

CFC/ozone issue, but there appears to be a consensus that enough is known to begin an orderly phase-out of CFC production. Chlorine from CFCs is proba-

bly a major contributor to the seasonal Antarctic ozone decreases. Small ozone decreases have occurred at high latitudes in the northern hemisphere during winter. Although the cause of these decreases is not established, there is a possibility that they could be due to chlorine, primarily from CFCs. The CFCs have atmospheric lifetimes on the order of 100 years and, hence, their concentrations will decay only slowly after emissions are stopped. This means action should be taken.

On the other hand, the ozone decrease that has been observed has occurred at high latitudes during winter and spring when a low sun angle results in exposure of plants and animals to very low UVb levels. This argues that actions taken now will be taken in ad- vance of potential harm to plants and animals.

There is a consensus within govern- ments and industry that protective measures should be taken, and progress is being made. Industry is aggressively developing options so that the essential needs served by CFCs will continue to be met safely. Du Pont’s goal is to develop options that will meet the needs of CFCs in a safe and environmentally acceptable manner while achieving a long-term reduction in atmospheric chlorine from the compounds meeting those needs.

Acknowledgments The atmospheric science community de- serves credit for developing the basis for consensus on this important issue. I would like to thank those at Du Pont, especially C. B. Catanach, who provided the esti- mates of CFC production and consumption and of alternatives forecasts.

Reading list Anderson, J. G. In Ozone Depletion, Green-

house Gases and Climate Change; National Research Council: Washington, DC, 1989. Farman, J. C.; Gardiner, B. G.; Shanklin,

J . D. Nature 1985, 314, 207-10. Lovelock, J . E. Nature 1971,230, 379. Molina, M. J. ; Rowland, F. S. Nature 1974,

“Present State of Knowledge of the Upper Atmosphere, 1988: An Assessment Report”; NASA Reference Publication 1208; 1988. World Meteorological Organization Global

Ozone Research and Monitoring Project; “At- mospheric Ozone 1985, Assessment of Our Understanding of the Processes Controlling its Present Distribution and Change”; Report Number 16; 1986.

249, 810-12.

Mack McFarland is a research associ- ate in the Freon Products Division of the Du Pont company in Wilmington, DE. He has been studying the science of atmospheric ozone since 1974 and is science coordinator of environmental programs for Du Pont. McFarland has a B.S. degree in chemistry from the University of Exas at Austin and a Ph.D. in chemical physics from the University of Colorado.

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Chlorofluorocarbons and ozone